Re: [PATCH, rs6000] Add support to __builtin_cpu_supports() for two new HWCAP2 bits

[Peter Bergner <bergner at vnet dot ibm dot com>]

On 6/27/17 11:07 AM, Segher Boessenkool wrote:
> Not use an installed header, that's not what I'm asking.  Share the
> source file, i.e., just copy it over from the glibc source tree (it
> should probably hold the master copy).  Fewer typos, cannot forget to
> update some entry, etc.

So the glibc file is:

  sysdeps/powerpc/bits/hwcap.h

which contains only the #define PPC_FEATURE[2]_* definitions.
The GCC file is:

  gcc/config/rs6000/ppc-auxv.h

and contains the same #define's as hwcap.h above, plus the additional
#defines's:

/* The PLATFORM value stored in the TCB is offset by _DL_FIRST_PLATFORM.  */
#define _DL_FIRST_PLATFORM             32

/* AT_PLATFORM bits.  These must match the values defined in GLIBC. */
#define PPC_PLATFORM_POWER4            0
#define PPC_PLATFORM_PPC970            1
#define PPC_PLATFORM_POWER5            2
...

which match values in glibc's sysdeps/powerpc/dl-procinfo.h, but that
file contains a lot more than just the defines that we (GCC) doesn't
want or need.

ppc-auxv.h also contains the following helper macros that calculate the
fixed offsets to the TCB slots that glibc initializes, but glibc has
access to the structs that the slows live in, so they don't need these
helper macros and hence don't have them:

/* Thread Control Block (TCB) offsets of the AT_PLATFORM, AT_HWCAP and
   AT_HWCAP2 values.  These must match the values defined in GLIBC.  */
#define TCB_PLATFORM_OFFSET ((TARGET_64BIT) ? -28764 : -28724)
#define TCB_HWCAP_BASE_OFFSET ((TARGET_64BIT) ? -28776 : -28736)
#define TCB_HWCAP1_OFFSET \
  ((BYTES_BIG_ENDIAN) ? TCB_HWCAP_BASE_OFFSET : TCB_HWCAP_BASE_OFFSET+4)
#define TCB_HWCAP2_OFFSET \
  ((BYTES_BIG_ENDIAN) ? TCB_HWCAP_BASE_OFFSET+4 : TCB_HWCAP_BASE_OFFSET)
#define TCB_HWCAP_OFFSET(ID) \
  (((ID) == 0) ? TCB_HWCAP1_OFFSET : TCB_HWCAP2_OFFSET)

These are only used in rs6000.c, so I could move them there.

So given the above, how do we want to handle this?  If we were to copy a
header file(s) over from glibc, are we able to modify it in the process?
Ie, to remove the parts we don't need like hwcap.h's use of:

  #if !defined(_SYS_AUXV_H) && !defined(_SYSDEPS_SYSDEP_H)
  # error "Never include <bits/hwcap.h> directly; use <sys/auxv.h> instead."
  #endif

which would trigger for our use of it.  And also to remove unneeded code from
dl-procinfo.h, since we only want the #defines.

Peter